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Review ArticleA Review on the Effect of Various Chemical Treatments on theMechanical Properties of RenewableFiber-Reinforced Composites

M. Aravindh,1 S. Sathish,1 R. Ranga Raj ,2 Alagar Karthick ,3,4 V. Mohanavel ,5

Pravin P. Patil,6 M. Muhibbullah ,7 and Sameh M. Osman8

1Centre for Machining and Materials Testing, KPR Institute of Engineering and Technology, Coimbatore 641407, India2Department of Aeronautical Engineering, Sri Ramakrishna Engineering College, Coimbatore 641 022, Tamilnadu, India3Renewable Energy Lab, Department of Electrical and Electronics Engineering, KPR Institute of Engineering and Technology,Coimbatore 641407, Tamilnadu, India4Departamento de Quimica Organica, Universidad de Cordoba, EdificioMarie Curie (C-3), Ctra Nnal IV-A,Km 396,E14014 Cordoba, Spain5Centre for Materials Engineering and Regenerative Medicine, Bharath Institute of Higher Education and Research,Chennai 600073, Tamilnadu, India6Department of Mechanical Engineering, Graphic Era Deemed to be University, Bell Road, Clement Town, 248002 Dehradun,Uttarakhand, India7Department of Electrical and Electronic Engineering, Bangladesh University, Dhaka 1207, Bangladesh8Chemistry Department, College of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia

Correspondence should be addressed to M. Muhibbullah; [email protected]

Received 20 November 2021; Accepted 24 December 2021; Published 25 January 2022

Academic Editor: Lubos Kristak

Copyright © 2022 M. Aravindh et al. 'is is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Increased environmental concerns and global warming have diverted effort all over the world to focus on renewable andsustainable resources for the next generation of composite products due to their recyclability, renewability, cost effectiveness, andsatisfactory mechanical performance. Bio/natural fibers which are environment friendly materials employed as reinforcementhave led to developing a biocomposite for reduction in greenhouse emission and carbon footprints. However, biofibers are alsohaving some limitations that need to be addressed including poor compatibility between the reinforcing fiber matrices, highmoisture absorption, swelling, poor chemical and fire resistance, and high dispersion of mechanical properties. A lot of researchhas been performed on physical and mechanical properties of natural fiber composite. Properties of such novel composite mainlydepend on adhesion between fiber and matrices. Consequently, poor adhesion, high moisture absorption, and swelling lead toformation of crack in both the matrix and fiber. 'erefore, numerous techniques have been tried till date to modify both fibersurfaces to enhance their adhesion and reduce their water absorption. 'is review article provides comprehensive informationabout effect of various surface modification techniques that include alkaline, silane, acetylation, permanganate, peroxide,benzoylation, acrylonitrile grafting, maleic anhydride grafted, acrylation, and isocyanate. In addition, the effects of cellulose,hemicellulose, lignin, and pectin of biofibers are also reported.'is review concluded that chemical treatment of biofibers with 5%NaOH concentration improves the physical, mechanical, and thermal properties of the resulting composites compared tountreated fiber composites.

HindawiAdvances in Materials Science and EngineeringVolume 2022, Article ID 2009691, 24 pageshttps://doi.org/10.1155/2022/2009691

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https://doi.org/10.1155/2022/2009691

1. Introduction

Presently, the automotive industries do face many chal-lenges, such as the shortage of supply of fossil fuels, newtechnological innovations, and environmental sustainabilityto fight global warming. 'e above listed factors increase thepressure in the current materials design and manufacturingtechnologies in automotive industries [1, 2]. A lightweightmaterial was a necessity for the structural components forlower energy consumption of vehicles. 'e research com-munity has shifted the focus towards effective utilization ofthe biofibers extracted from renewable sources. 'e naturalfibers are being employed for application components suchas interior dashboard trims for automobiles and householdapplications [3, 4]. 'e utilization of lightweight and low-cost biofibers such as abaca, banana, bamboo, coir, flax,ramie, tea leaf, pineapple, sisal, kenaf, and jute was alreadytried as reinforcements in the polymer matrix for compositematerials used for manufacturing automobile components[5–7]. 'e biodegradability of biofibers is associated withphysical, chemical, mechanical, thermal, and moistureconditions which have increased their scope of use in nu-merous applications [8, 9]. Based on the usage and utili-zation, usually biofibers are classified as primary andsecondary fibers. Primary fibers include jute, sisal, kenaf, andhemp fibers, where these fibers are grown for their fibercontents. Secondary fibers include agroresidues, coir fibers,and pineapple fibers, which are fibers obtained from plantby-products. 'ese fibers can be further classified into sixtypes: bast fibers (flax, jute, etc.), leaf fibers (sisal, pineappleleaf fiber), seed fibers (cotton, coir), core fibers (hemp,kenaf), grass and reed fibers (wheat, corn, and rice), and allother types (wood and roots). 'e structure and chemicalconstituents of biofiber depend on several factors like ex-traction process, location of plant growth, climate, plant age,and plant nature [10–13]. When compared to synthetic glassfibers, natural fibers have better specific modulus. 'e costsaving on the material owing to the use of plant based fibersand nonabrasive nature of the materials during mixing andmoulding makes the natural fibers a promising reinforce-ment for polymer composites [14]. 'ese advantages makethe biofiber be employed in any application like automobile,domestic utilities, and distinction [15, 16]. 'e biofibercomposites were used in many places because of the fol-lowing merits: eco-friendly nature, easy availability, lowweight, good strength, low cost, and ease of manufacturingprocess [17–19]. 'e foremost disadvantages of the biofibersare that they absorb more moisture. 'e moisture absorp-tion of biofibers has several unfavourable effects on theirproperties and thus affects the long-term performance of thecomposites [20]. 'e mechanical and thermal performancesof bio/natural fibers reinforced composites (NFRC) wereinfluenced by weight/volume fraction of fiber, fiber orien-tation, selection of chemical treatment method, and physicalcharacteristics of the natural fiber [21–23]. However, thewater absorption and thermal stability of the compositelaminate were noticed to reduce with increase in weightfraction of fiber [24–28].'e natural fibrils are constituted ofcellulose, hemicellulose, and lignin in varying percentages.

In addition, the natural fibers do have other substances likepectin, wax, and other water soluble compositions. 'ecellulose is enclosed in soft lignin, while hemicellulose formsthe ancillary layer of the fiber material [29]. Ishak et al. [30]studied the tensile strength of bagasse fiber obtained fromplants of different heights. 'e tensile properties werespecifically high for the fibers extracted from bottom of treedue to their chemical composition, particularly cellulose,hemicellulose, and lignin. Moisture absorption of the naturalcomposites was increased when fiber content increasedbecause of its higher cellulose content amount. George et al.[31] stated that the biofibers have high amount of hydro-philic property which leads to poor adhesion propertiesbetween the hydrophilic fiber and hydrophobic matrix.Hydrophilicity is the stronger affinity of the fibers towardsmoisture. 'e major disadvantages of natural fiber are pooradhesion between fiber and matrix, presence of cellulosecontent, moisture absorption, and voids at interface betweenfiber and matrix which results in dimensional inaccuracy,thus affecting the mechanical properties [32–35]. In additionto that, presence of high moisture content in fiber leads toswelling of fiber and matrix within composites resulting indimensional instability. 'is disadvantage and limitationcan be overcome by chemical treatments. 'e chemicaltreatments are carried out to reduce the hydrophilic natureof fiber but the surface treatments not only modify the fibersurface but also increase the fiber strength leading to theimprovement of adhesion between fiber and matrix [36–38].'e most common chemical treatment and surface treat-ment methods are silane, alkaline, acetylene, maleatedcoupling, anhydride, and benzolytation. Optimization offiber and matrix is aimed to improve the adhesion, surfacetension, interfacial strength, and wettability that offer goodsurface roughness leading to good bond [39]. 'ese could bedone by adding suitable compatibilizer/coupling agent andchemically treating the fiber. Recent literature surveyshowed that several works have been published on differentnatural fibers extracted from renewable sources which areused as reinforcement with polymer matrix over a widedimension of applications. 'is survey depicts varioussurface treatments done to enhance the properties of thefibers and for enrichment in mechanical properties of thecomposites in contrast with untreated fiber-reinforcedpolymer composites. 'erefore we summarized the majorfindings on different types of chemical treatments andsurface treatments.

2. Constitution of Biofiber

2.1. Cellulose. In plant based fibers, cellulose is the primarystructural constituent and the cellulose portion influencesthe mechanical properties of the lignocellulose fibers. 'ecellulose is the fundamental constituent which is liable forthe strength of the plant fibers and variance in strength maybe due to growth conditions of the plant and soil nature.Cellulose is a lined, semicrystalline polysaccharide build-upof polymer links compromising the recurring modules ofanhydroglucose grouped via 1,4-β-D-glucosidase. 'e re-curring modules of the monomers are termed as degree of

2 Advances in Materials Science and Engineering

polymerization. 'e monomers of glucose in the chain ofcellulose result in formation of the hydrogen bonds amongthe link forming fibers as well as associated chains. 'ebinding of intramolecular and intermolecular hydrogenregions results in development of linear crystalline structurecalled cellulose.

2.2. Hemicellulose. Hemicellulose is a multidiverged poly-saccharide made up of many distinctive glucose monomers,while cellulose is composed of only single 4-βD-glucopyr-anose repeating units. When compared to cellulose, theconstituent of hemicellulose varies from one plant to other.'e amorphous nature of the hemicellulose is confirmed byits high degree of chain grouping.

2.3. Lignin. Lignin is an intricate hydrocarbon polymercomposed of aliphatic and aromatic elements. In addition tothe cellulose, lignin is another important constituent in thelignocellulose fibril. 'e lignin binds the fibers together tomake the fiber surface be stiffer enough by giving com-pression strength to the plant. 'e chemical composition oflignin is made of phenylpropane elements obtained from anenzyme-initiated dehydrogenate polymerization of threedistinct constituents which are trans-p-coumaryl, trans-coniferyl, and trans-sinapyl.

2.4. Pectin. Pectin is an element of acidic polysaccharideswith complex structure. 'e main constituents of pectin arehomopolymeric acid and partial residues of methylatedpoly-α-(1-4)-D-galacturonic acid. When treated with alkalior ammonium hydroxide, pectin will become a water solublepolymer. 'e role of pectin is to function as cementingelement among the plant fibrils which binds with otherconstituents to form stacks. Higher amount of pectin ispresent in the primary cell wall and the middle lamella oflignocellulosic fibril. During the process of retting, most ofthe pectin contents get removed from the natural fiber. Onlyafter the removal of pectin does the natural fiber get qualifiedto be employed as reinforcement material with polymermatrices. Pectin is another component which makes thecellulose fibers get attached to all other constituents of thefiber. When compared to cellulose, lignin and pectin are theweaker amorphous polymers [40].

3. Effect of VariousChemical Treatments on theMechanical Properties of Composites

Kobayashi et al. [41] discussed the mechanical properties ofthe hemp fiber-reinforced composite fabrication, and thehemp fiber was chemically treated to improve compatibilitybetween fiber and the matrix. 'e authors found that thephysical and mechanical properties of the natural andsynthetic fibers were influenced by climate/natural/envi-ronmental changes. Hence, the surface of the hemp fiber wastreated by chemical treatment process like acetyl, alkali, andsilane. Alam et al. [42] explored the tensile strength of a newcomposites, combined with untreated kenaf, treated kenaf,

jute fiber, and jute rope. 'ey observed that the tensilestrengths of kenaf and jute fiber are higher than that of juterope. Similarly the water absorption properties of treatedfiber were higher than those of untreated fibers. 'e stresstransfer capacity of the fiber gets improved as a result ofmicro void exclusion and the fiber surfaces turn moreuniform.'e diameter of the fiber is also improved owing tothe axial splitting of the fibrils [43]. Wang et al. [44] studiedthe feasibility of using coffee hull as reinforcement memberwith high density polyethylene matrix. Improvement inmechanical characteristics of the coffee hull polyethylenecomposites was compared against various chemical modi-fications done on the coffee hull powder. It was found thatcoffee hull powder subjected to calcium hydroxide treatmentresulted in maximum strength of the composites. 'e fiberloading was found to increase up to 10 wt% above which thetensile strength decreases, whereas the flexural strength wasfound to be prominent for coffee hull treated with maleicanhydride grafted polypropylene. 'e moisture absorptionproperty was found to be significant for the compositesubjected to calcium hydroxide treatment. 'erefore it wasconcluded that coffee hull powder could be a possible al-ternate for synthetic fiber. Rout et al. [45] conductedmorphology analysis of palm tree leaf stalk fibers, where theSEM images confirmed the removal of wax, oil, andhemicellulose content from treated fibers. 'e cleanersurfaces besides pores were noticed in treated fibers com-pared to untreated fibers. 'e natural plant fibers have manyadvantages; there are also a few limitations which have to bestudied. 'e major limitation of natural plant fibers is theirhydrophilic nature which restricts the use of the fibers asreinforcement in PMC. 'e inappropriateness between thehydrophilic fibers and hydrophobic matrix results inswelling due to moisture absorption and it shows the poorinterfacial bonding between matrix and the fibers [46–48].Improvement was found in the interfacial bonding betweenthe fiber and matrix due to the chemical treatments of fiberswhich also reduce the hydrophilicity, fiber surface cleanness,the moisture absorption, and improvement in the surfaceroughness [49]. Various natural fiber surface treatments likealkaline, silane, acetylation, and preimpregnation withpolyethylene solution resulted in the enhancement ofstrength due to increase of interfacial bonding between fiberand matrix [50]. Venkatesha Gupta et al. [51] developed anew composite material which had the highest strength toweight ratio in comparison to existing composite materials.Sisal and hemp fibers were reinforced with epoxy matrixprepared using compression moulding method according toASTM standard. For alkali treatment, NaOH (sodium hy-droxide) was used and the amount of reinforcement waschanged from 10% to 50% by weight. After the specimen wasprepared, various mechanical properties were investigatedand it was proved that the prepared specimen was better interms of mechanical properties. Athipathi and HegdeSowmitha Vijay [52] discussed the experimental evaluationbased mechanical properties of coir and Roystonea regia-epoxy laminate with various fiber contents ratios. Orien-tation of the fiber was maintained as 0°, 45°, and 90°. Fromthe results, three different points were observed. 'e

Advances in Materials Science and Engineering 3

untreated matrix-material based composite exhibits hightensile strength, high flexural strength, and high impactstrength. 'e fibers were subjected to 30% NaOH solutiontreatment for 1 h.'emechanical and electrical properties ofcomposites with treated fibers were compared with those ofuntreated fiber composites. 'e modification of plant basedlignocellulose fibrils by sodium hydroxide is the most widelyadopted technique to alter the cellulose molecular portion ofthe natural fiber.'e coir/epoxy composites were used in theseat cushions, mirror casing, storage tank, post boxes,helmet casing, brushes, ropes, bags, brooms, door shutters,and building panels. 'e alkali treatment results in for-mation of amorphous region from the densely packedcrystalline cellulose structure. 'e alkali-sensitive hydroxylunits existing in the natural fibers were removed, whichfurther react with the water molecules. 'ereby the moistureresistance property of the fiber improved. 'e alkali treat-ment also removes some portions of hemicellulose, lignin,pectin, wax, and other surface related impurities present inthe natural fiber [53, 54]. 'ereby the effective bondingbetween the fiber and the matrix is also enriched and themechanical and thermal properties of the composites areimproved. When the percentage of alkali treatment is in-creased, excess delignification of the natural fiber occurs,which leads to damage of fibers, and the mechanicalproperties of the fibers get reduced [55]. Alkalized ligno-cellulose fibers have reduced lignin content, partial removalof wax and oil covering substances happens, and disinte-gration of crystalline cellulose occurs.

More research articles have been published on the in-fluence of mercerization on the mechanical and thermalproperties of lignocellulose fiber-reinforced polymer matrixcomposites [56, 57]. Sathish et al. [53] explored the influenceof mercerization of date palm fibers on the mechanical,thermal, and morphological properties. It was reported that5% of NaOH concentration enhanced mechanical andthermal behaviour of the composites. When the percentageof sodium hydroxide was increased to 10%, deterioration onthe properties was observed owing to the damage of fibers atincreased concentration. 'e thermal resistance of the fiberwas also improved due to the pulling out of waxy layers andvarious surface impurities present in the date palm fibers.Chen et al. [58] investigated the wettability and thermalstability of bamboo fibers exposed to alkali treatment. 'epercentage of alkali treatment was varied; it was found thatsurface roughness of the bamboo fiber was increased andfound to be optimum at 15% of NaOH treatment. 'ethermal stability and wettability were also found to bepromising for the similar alkali concentration. Reddy et al.[59] investigated the tensile and structural properties ofborassus fruit fine fibers subjected to 5% of alkali treatmentunder different treatment time. 'e crystallinity of the fiberswas analysed by X-ray diffraction technique. 'e removal ofamorphous hemicellulose substance was witnessed by FTIRanalysis. 'e concentration time of 8 hours resulted inoptimum fiber properties. It was also found that the borassusfibers will be a suitable reinforcement for the manufacturingof green composites. Balaji and Nagarajan [60] investigatedthe tensile and chemical behaviour of cellulose fibers

extracted from saharan Aloe vera cactus leaves exposed tomercerization treatment. 'e fibers exposed to merceriza-tion resulted in removal of hemicellulose, lignin, wax, andother surface related impurities present in the fibers. It wasalso found that the hydrophobic nature of the lignocellulosefibrils is lowered and interfacial adherence among the fibrilsand the matrix enhanced. Increase in thermal stability of thefibrils was witnessed by TG analysis. 'e SEM analysis alsoconfirmed the removal of hemicellulose, lignin, wax, andother layers present in the fibers. Finally, it was found thatnatural fiber extracted from saharan Aloe vera cactus leaveswas found to be a suitable alternate to synthetic fibers forreinforcement with polymer matrix. Dawit et al. [61] ex-plored the property of Acacia tortilis fibrils extracted fromthe barks of Acacia tortilis tree. 'e extraction of fiber wasbased on natural water based retting. 'e extracted fiberswere subjected to mercerization treatment with 10% and20% of NaOH solution. 'e alkalization of the fiber resultedin removal of hemicellulose, lignin, wax, and other surfacerelated impurities present in the fiber. As the percentage ofalkali treatment is increased to 20%, decrease in tensilestrength of the fibrils was noted. 'is phenomenon could beexplained as follows: when the concentration of alkali so-lution exceeds the limit, the diameter of the fiber gets re-duced even further, which results in reduced tensile strength.It was also concluded that Acacia tortilis fiber could be aviable alternate for manmade fiber in polymer compositeapplications. Narayanasamy et al. [62] explored the possi-bility of lignocellulose fibril extracted from Calotropisgigantea fruit bunch as a possible alternate for artificial fiber-reinforced polymer composites. 'e fibers were extractedfrom the fruits of Calotropis gigantea fruit bunch throughretting process. 'en the extracted fibers were mercerizedwith 5% of NaOH. XRD and FTIR analysis revealed theremoval of hemicellulose, lignin, wax, and other surfacerelated impurities existing in the fibers. 'e thermal stabilityof the fibrils was also enriched by the influence of alkalitreatment, which is inferred by TG analysis. Finally SEManalysis revealed that the surface of fibers was rougher due toalkali treatment. Negawo et al. [63] researched the effect ofalkali modification on the Ensete stem fibrils obtained fromthe Ethiopian Ensete ventricosum plant. 'e fibers weresubjected to 2.5%, 5%, and 7.5% of mercerization. 'emercerization resulted in removal of lignin, wax, andhemicellulose existing in the fibrils.'e 5%mercerized fibersexhibited better properties when compared with untreatedfibrous composites owing to the enhanced interfacial ad-herence between the fiber and the matrix. 'e 5% alkalizedfibers exhibited better mechanical properties under staticand dynamic conditions. From the experimentation, it wasalso concluded that Ensete stem fiber could be a possiblealternate to synthetic fiber-reinforced polymer compositesfor wide assortment of applications. Reddy et al. [64] studiedthe influence of NaOH and KOH fiber surface modificationson the mechanical properties of Tapsi fiber-reinforced epoxycomposites manufactured by hand layup technique. Initiallythe fibrils were pretreated with 5% of concentrations for twohours. Composites were manufactured by varying the fiberweight fraction. Tensile and flexural test performed on the


composite samples revealed that composites with 15% offibers exhibited higher properties for NaOH-treated fiberswhen compared to KOH-treated fibers. FTIR analysisrevealed the removal of functional groups present in thefibers and XRD analysis revealed the improvement ofcrystallinity index and size for NaOH-treated fiber-rein-forced composite samples when compared to KOH-treatedsample. SEM analysis revealed fiber pull-out as a result ofimproper fibril wetting, which resulted in poor adherencebetween the interfaces of fiber and matrix in the composites.Senthilkuamr et al. [65] reviewed the mechanical propertiesof sisal fiber-reinforced polymer composites. 'e sisal fibershave been extracted by the process of decortication and theextracted sisal fibers were exposed to chemical treatment likealkalization and coupling agents. 'e pretreated sisal fibershowed improvement in mechanical, hydrophilic tendencyresulting in effective bonding between the interfaces of fiberand polymer matrix. 'e mechanical properties of the de-veloped composites depend on different characteristics likefiber length, fiber orientation, fiber volume fraction, andseveral other parameters. Appreciable enhancement inmechanical properties was found owing to the chemicalmodifications on the surface of sisal fibers. Overall, it wasconcluded that the enrichment in properties of the sisalfiber-reinforced composites depends on surface treatmentconcentration type and time; beyond the concentration level,deterioration in properties of the fibers was noticed. It wasalso concluded that impact strength of the sisal fibercomposites decreased as a result of chemical modificationsdone on the sisal fibers. Balaji et al. [66] explored the in-fluence of fiber content on the mechanical properties of thealkali-treated bagasse fiber reinforced with cashew nut shellliquid. Initially the fibers were chopped to 10 and 20mm andthe composites were prepared by compression mouldingtechnique by varying the fiber volume fraction as 0, 5, 10, 15,and 20 wt%. 'e tensile and flexural test revealed thatmaximum strength was attained for 15 wt% of fiber-rein-forced composites. FTIR analysis revealed the removal offunctional groups present in the fibers owing to the alkalitreatment of sugarcane bagasse fiber.'e thermal stability ofthe fibers was also enhanced due to the mercerization andSEM analysis revealed the enhanced interfacial adherencebetween the fiber and the matrix which resulted in thehomogeneous nature of composite. Komal et al. [67] in-vestigated the prominence of alkalization done on thesurface of banana fibers. 'e surface modified banana fiberswere reinforced with polypropylene and then the tensile,flexural, and impact strength and degradation behaviourwere studied for untreated and surface modified bananafiber-reinforced polypropylene composites. 'ermogravi-metric analysis revealed removal of hemicellulose, lignin,pectin, wax, and other surface related impurities present inthe banana fiber as a result of mercerization. Significantenhancement in tensile and flexural strength was foundbetween untreated and pretreated banana fiber-reinforcedpolypropylene composites. Improvement of impact strengthby 11.5% was observed for untreated and mercerized bananafiber-reinforced polypropylene composites. 'e testedcomposite samples were subjected to morphology analysis

by scanning electron microscope to study the fracture be-haviour of the composite samples. Fiber pull-outs wereobserved in untreated banana fiber-reinforced polypropyl-ene composite; as a result, tensile and flexural strength of thecomposites decreased in contrast with alkali-treated anduntreated banana fiber-reinforced polypropylene composite.Marginal reduction in weight loss of the samples was alsoobserved. Alkali-treated banana fiber-reinforced polypro-pylene composites absorbed less water as the hydrophilictendency of the fiber was altered by alkali treatment, whereasthe untreated banana fiber-reinforced polypropylene com-posites absorbed water due to higher hydrophilic tendency.Ameer et al. [68] explored the mechanical and moisturecharacteristics of hydrophilic modified jute fiber-reinforcedunsaturated polyester composites. 'e extracted jute fiberwas exposed to mercerization treatment to improve thehydrophilic tendency. Better interlocking in the middle offiber and matrix was attained by the removal of amorphoussubstances present in the jute fibers. 'e mercerizationtreatment resulted in significant reduction in hydrophobicnature of the jute fiber. 'e mercerized jute compositesshowed improved mechanical properties in contrast withuntreated jute composites. Chin et al. [69] investigated themechanical and thermal characteristics of bamboo fiber-reinforced composites, where the bamboo fibers were ex-posed to mercerization with different concentrations overvarying time. 'e effect of alkali treatment was inferredwith FTIR and XRD analysis. Enhancement in crystal sizeand crystallinity index was observed by XRD analysis andremoval of lignin, cellulose, and other surface relatedimpurities was observed by FTIR analysis. 'e thermalstability of the composites was also enhanced, which wasevident from the thermogravimetric analysis. 'e com-posites with 40 wt% contributed to maximized tensile andflexural properties of the composites. Balaji et al. [66]explored the mechanical properties of sugarcane bagassefiber-reinforced cardanol composite. Mercerization of fiberresulted in removal of amorphous substances and im-proved interlocking between the bagasse fiber and cardanolmatrix. 'e composites with 15 wt% of fiber exhibitedbetter properties. Senthamaraikannan et al. [70] exploredthe possibility of using Acacia planifrons fibers as possiblereinforcement material with polymer matrices. 'e fiberswere extracted by the process of retting.'e extracted fiberswere subjected to mercerization with varying percentage ofsodium hydroxide. It was found that the mercerizationleads to improvement of crystallinity index and thermalstability and removal of amorphous substances present inthe fibers. 'e optimal alkali treatment is found to be 5%.Tables 1–10 present the effects of various chemical treat-ments of biofibers.

Mouhoubi et al. [174] reported the SEM images of alfafiber with alkali treatment (5% NaOH) at different timeintervals (2 h, 4 h, 6 h, and 24 h). Figures 1(a)–1(e) representthe alfa fiber. Figure 1(b) shows the fiber treated with 5%NaOH at 2 h. During this time period, the waxy substancesin the fiber were removed. Figures 1(c)–1(e) show that thefiber resulted in low moisture absorption, removal of ex-tractives, and increase in crystallinity and stiffness.


Table 1: Effect of alkaline treatment on various biofibers.

S.no. Fibers used Type of chemical treatment Effects Ref.

1 Kenaf Alkaline treatment At 6% concentration of NaOH it has good effect on kenaf fiberresulting in removal of all impurities from the surface. [71]

2 Bamboo, kenaf, hemp,sisal, jute, and kapok Alkaline treatment

'e treatment removed the noncellulose constituent in fiberssuch as lignin, wax, and oils, promoted ionization of hydroxylgroup of cellulose to alkoxide, and reduced the hydroxyl

group content.'e treatment improved the surface roughnessand hydrophobicity resulting in good adhesion.

[72]

3 Pineapple leaf Alkaline and acetic Improvements in tensile strength, impact strength, andflexural strength. [73]

4 Abaca Alkaline and silane treatment 'e silane-treated fiber has higher thermal transfer coefficient. [73]

5 Bamboo Alkaline treatmentAn enhancement in tensile strength by adding 30% treated

bamboo which is slightly higher than silane-treatedcomposite.

[74]

6 Sisal Alkaline treatment Improvement in interfacial shear strength. [75]

7 Sisal/hemp Alkaline treatment At 10% concentration of NaOH, enhanced the flexuralstrength by adding 40 wt% sisal and hemp. [75]

8 Curaua Alkaline treatment An increase in NaOH concentration and decrease of fiberdiameter, fiber density, and fiber weight. [76]

9 Ramie Alkaline treatment Alkali treatment possesses better tensile strength than silane-treated fiber composite. [76]

10 Hemp Alkaline treatment 'e treated fiber has high crystallinity resulting inimprovement in tensile strength and Young’s modulus. [76]

11 Jute Alkaline treatment 'e treatment removed hemicellulose, pectin, and ligninresulting in decreased fiber diameter. [76]

12 Basalt Alkaline treatment 'e NaOH-treated fiber has superior properties compared toglass fiber. [76]

13 Banana Alkaline treatment 5% NaOH-treated fiber has better properties. [77]14 Luffa/coir Alkaline treatment Improvement in tensile and flexural strength and hardness. [78]

15 Luffa/groundnut fiber Alkaline treatmentAn increment in mechanical properties by removal of

hemicellulose, wax, lignin, and impurities from the fibers,thus increasing the adhesive characteristics of composite.

[79]

16 Abaca Alkaline treatment Improvement in moisture resistance. [80]

17 Alfa Alkaline treatmentAt 10% of NaOH content, increases in flexural strength andflexural modulus by 60% and 62%, respectively, and fiber

becomes stiffer and brittle.[81]

18 Drumstick (Moringaoleifera) Alkaline treatment 'e addition of glass fiber increased impact strength and

frictional coefficient. [82]

19 Ladies finger Alkaline treatment

Double-stage chemical treatment possessed better propertiesthan single-stage treatment, while an increase in span length

decreased the tensile strength and increased Young’smodulus.

[83]

20 Tamarind Alkaline treatment Chemically treated 2 cm fiber length was optimum to achievebetter hardness, impact, and frictional coefficient. [84]

21 Vetiveria zizanioides/jute Alkaline treatment

'e treated fibers improved tensile strength, flexural strength,and impact strength by 26.8%, 30.44%, and 59.1%,

respectively.[85]

22 Borassus Alkaline treatment At 5% of NaOH content, significantly increased tensileproperties. [86]

23 Palm wood Alkaline treatment 'e optimum residual mass at 0% to 0.75% NaOH. Withfurther 1% NaOH it decreased. [87]

24 Palmyra palm leaf stalkfiber (PPLSF)/jute Alkaline treatment 'e alkali-treated PPLSF has maximum tensile and flexural

properties by the addition of alkali-treated jute fiber. [88]

25 Roystonea regia Alkaline treatment Improvement in tensile and flexural properties. [88]

26 Borassus flabellifer(Asian palmyra) Alkaline treatment Improvement in tensile strength. [88]

27 Buriti and ramie Alkaline treatmentAt 2% NaOH treatment of ramie fiber, increased flexural

strength by 70%. However, alkali treatment was only favorablefor buriti fibers.

[89]


Table 1: Continued.


28 Rice husk Alkaline treatment

An increase in cellulose content, resulting in increasedcrystallinity index. 'erefore diameter decreased from 170 to7mm, as well as further diameter value from 10 to 15 nm by

performing acid hydrolysis treatment.

[90]

29 Rice husk Alkaline treatment Improvement in adhesion characteristics. [90]

30 Jute Alkaline treatment An increase in flexural strength, modulus, and interlaminarshear strength. [91]

31 Coir Alkaline treatment At 5% alkali treatment increases in impact and flexuralstrength for 72 h by 40%. [92]

32 Jute Alkaline treatment At 5% alkali treatment increases in flexural strength for 4 h by20%. [93]

33 Banana Alkaline treatmentAt 1% alkali treatment enhanced flexural strength, flexuralmodulus, tensile strength, and tensile modulus by 20, 12, 132,

and 131%, respectively.[94]

34 Ramie Alkaline treatment At 9% alkali treatment enhanced tensile strength for 1 h by23%. [95]

35 Jute Alkaline treatmentAn increase in flexural strength, flexural modulus, andinterlaminar shear strength by 35%, 23%, and 19%,

respectively.[93]

36 Abaca/roselle Alkaline treatment'e treatment increased fiber/matrix adhesion property dueto removal of hemicellulose, waxes, lignin, and impurities

from the fibers.[96]

37 Jute Alkaline treatment

'e treatment removed the hemicellulose and promoted theinterlocking points in the fiber for better adhesion and stresstransfer across the interface resulting in increased tensile

strength, flexural strength, flexural modulus, and interlaminarshear strength.

[97]

38 Jute Alkaline treatment'e treatment increased the cellulose content after removal ofpectin, lignin, and other impurities. An increase in cellulose

content leads to better interfacial adhesion.[98]

39 Sisal Alkaline treatment 'e treatment had better mechanical properties due to goodadhesion between fiber and matrix. [99]

40 Oil palm Alkaline treatment A bigger increase in flexural strength by performing 24-hourNaOH treatments compared to other chemical treatments. [100]

41 Jute Alkaline treatmentAt 25% fiber loading and 10% NaOH treatment showed

increase in tensile strength due to decrease in fiber diameterand density.

[101]

42 Jute Alkaline treatmentAt 20% fiber loading and 10% NaOH treatment showed

increase in tensile strength due to decrease in fiber diameterand density.

[102]

43 Napier grass Alkaline treatment

'e 12 h soaking time of treated fiber had least fiber diameterand mass. 'e 6 h soaking time exhibited highest tensile

strength. An increase in surface roughness with the increase insoaking time beyond 18 h. However, 24 h-treated fiber had

damage on its surface.

[103]

44 Henequen Alkaline treatment 'e treated fiber had higher adsorption rate at 100 h to attainadsorption equilibrium. [50]

45 Sisal Alkaline treatment'e 45min of treatment yielded more level of crystallinitywith more cell wall structure. Tensile and shear strength were

increased by 12.04% and 173%, respectively.[104]

46 Sisal Alkaline treatmentAn increase in crystallinity decreased the absorption rate.Optimum fiber length 5.8–9 cm displays better performance

in tensile strength with increase in fiber loading.[105]

47 Ladies finger Alkaline treatment Removal of hydrophilic hemicellulose led to enhanced surfaceroughness. [83]

48 Kenaf Alkaline treatment Chemically treated 6%NaOH sample was optimum to achievebetter tensile strength and modulus of elasticity. [106]


Figure 2 represents the SEM images of untreated andtreated Prosopis juliflora fiber-reinforced epoxy compositesat different concentrations (5%, 10%, and 15%). Figure 2(a)shows the untreated fiber surface which consists of impu-rities and fiber pull-outs on the surface. 'is was due to thewaxy substances present on the surface of the fiber and theexistence of hydroxyl groups, which leads to water ab-sorption, weakening interfacial strength with the matrix.

Figures 2(b)–2(d) represent the treated Prosopis juliflorafiber-reinforced epoxy composites at concentrations of 5%,10%, and 15%, and increase in alkali treatment beyond 5%damaged the fiber surface and reduced the cellulose contentin the fiber, which in turn resulted in lower strength andstiffness [175].

Liu et al. [176] researched the supremacy of silanecoupling agent treatment done on the surface of corn stalk

Table 1: Continued.


49 Kenaf Alkaline treatment

At 9% NaOH alkali treatment displayed cleanest surfacealthough tensile strength decreased. However, 6% NaOHalkali treatment with higher temperature was optimum in

cleaning fiber.

[49]

50 Banana Alkaline treatment An enhancement in tensile modulus and impact and tensilestrength by adding 3 wt% of fiber. [107]

51 Banana Alkaline treatment At 10% of NaOH content, significantly increased thermalconductivity. [108]

52 Banana Alkaline treatment At 4% concentration of NaOH, enhanced the tensile strength,tensile modulus, and flexural strength. [109]

53 Banana Alkaline treatmentAlkali treatment possesses better tensile strength and flexuralstrength when compared with benzoylation and PSMA

treatment.[110]

54 Banana Alkaline treatment 'e treatment decreased modulus of rigidity, tensile strength,and strain due to degradation of cellulose. [111]

55 Pineapple leaf Alkaline treatmentAn increase in fiber density, cellulose, and crystallinity led to

enhanced tensile strength, thermal stability, and waterretention with increasing the NaOH up to 7% concentration.

[112]

56 Pineapple leaf Alkaline treatment

'e treated fiber significantly improved the flexural strength,impact strength, storage modulus, and thermal resistance by79%. Heat deflection temperature (171.3°C) which is close tothe melting temperature of neat polymer. Reduction in

crystallization by 14°C.

[113]

57 Pineapple leaf Alkaline treatment An enhancement in Young’s modulus by 30% compared tountreated fiber. [114]

58 WSF Alkaline treatment An enhancement in thermal stability by adding 3% NaOH. [115]59 Banana Alkaline treatment At 1% NaOH treatment possess better properties. [94]

60 Hemp 5% NaOH, 0.5% silane'e combined NaOH and silane treatment increased the

tensile and flexural strength by 100% and 45%, respectively.But fracture toughness decreased.

[116]

61 Jute Alkaline treatment At 4% NaOH treatment increased tensile strength up to 30%. [117]62 Agave Alkaline treatment Increased the fiber matrix adhesion and fracture strain. [118]

63 Palm leaf stalk/jute Alkaline treatment An increase in storage modulus and loss modulus by additionof jute fiber. [88]

64 Coir Alkaline treatment Enhancement in mechanical properties, moisture resistance,and adhesion properties. [119]

65 FlaxBenzoylation, peroxide,mercerization, silane

treatment.

'e treatment exhibited improved mechanical and physicalproperties. [120]

66 Hemp/jute Alkaline treatment Increase in crystallinity can enhance the fiber strength. [121–123]

67 Hemp Alkaline treatmentAn increase in crystallinity of PLA matrix due to crystallinecellulose in the alkaline-treated hemp fibers, which actsnucleating sites resulting in increase in fiber strength.

[124]

68 Kenaf/hemp Alkaline treatment 'e treated fiber found to have better mechanical properties,thermal stability, and moisture resistance. [125, 126]

69 Sisal Combined NaOH + actylation Increase in mechanical properties due to better adhesionbetween fiber and matrix. [104]

70 Tridax procumbens Alkaline treatmentAt 5% concentration of NaOH, enhanced the wettability and

crystallinity and reduced amorphous region and fiberdiameter.

[127]


Table 2: Effect of silane treatment on various biofibers.

S.no. Fibers used Type of chemical

treatment Effects Ref.

1 Kenaf Silane treatment'e presence of lignin and hemicellulose was removed by performing silanetreatment. Removal of lignin and hemicellulose led to enhanced interfacial

bonding.[71]

2 Pineappleleaf Silane treatment 'e treated fiber has fewer voids on the interface which makes strong

interfacial bonding and results in better mechanical properties. [71]

3 Abaca Mercerization and silanetreatment 'e silane-treated fiber has higher thermal transfer coefficient. [73]

4 Bamboo Silane treatment An enhancement in tensile strength by incorporation of 30% treated bamboo,while flexural strength is higher than that of NaOH-treated fiber. [74]

5 Sisal Silane treatment 'e treatment enhances the mechanical properties and moisture resistance. [75]

6 Hemp/kenaf Silane treatment 'e treatment possesses higher flexural modulus in comparison with alkali-

treated composite and similar to glass fiber composite. [128]

7 Hemp Silane treatment Flexural and tensile strength were increased by 2% and 4%, respectively. [129]

8 Kenaf Silane treatment An enhancement in storage modulus and viscoelasticity by 45% and 25%,respectively. [130]

9 Oil palm Silane treatment Reduced the mechanical properties due to poor adhesion between fiber andmatrix. [131]

10 Henequen Silane treatment An enhancement in tensile strength from 21MPa to 27 MPa by performingcombination of silane and NaOH. [50]

11 Sisal Silane treatment 'e treated fiber had higher impact strength compared to alkali-treated fibers. [132]

12 Banana Silane treatment An increase in flexural strength about 160% and considerable increase intensile and toughness. [133]

13 Banana Silane treatment An enhancement in impact and tensile strength by 30.84% and 19.43%,respectively, and slight increase in tensile modulus. [134]

14 Jute Silane treatment An increase in strength and modulus about 12% and 7% by alkali treatmentfollowed by silane treatment. [135]

15 Jute Silane treatment At 0.3%, silane-treated composites enhanced the tensile, flexural, andinterlaminar shear strength by 40%, 30%, and 55%, respectively. [113]

16 Pineappleleaf Silane treatment Improvement in flexural modulus and storage modulus by 47% as compared to

alkali treatment. [114]

17 Pineappleleaf Silane treatment 'e resulting composite has less Young’s modulus than alkali-treated

composites. [136]

18 Pineappleleaf Silane treatment Reduction of hydrophilic tendency of the fibers leads to increase in tensile

strength and crystallinity size but % crystallinity decreases. [72]

19 Hemp Silane treatment Found maximum mechanical properties compared to other chemicaltreatments. [137–139]

Table 3: Effect of acetylation treatment on various biofibers.



1 Coir/oil palm Acetylationtreatment

A bigger increase in tensile strength, flexural strength, Young’s modulus, andimpact strength by performing acetylation treatment compared to silane

treatment.[140]

2 Flax Acetylationtreatment An enhancement in tensile and flexural strength by 35%. [141]

3 Abaca Acetylationtreatment

'e treatment possessed higher tensile strength, Young’s modulus, and impactstrength by 81, 70, and 8%, respectively. [133]

4 Oil palm Acetylationtreatment

'e treatment has high strain value which resulted in enhanced elastic andimpact property. [100]

5 Green flax Acetylationtreatment

At 65% of relative humidity, decrease in the moisture absorption. An increase inthermal stability with increase in degree of acetylation. About 25% improvement

in strength properties was observed compared to untreated composites.[139]

6 Flax Acetylationtreatment

At 18% of acetylation, concentration of flax fiber exhibited better tensilestrength and thermal stability by 25% and 50%, respectively. However, the

addition of maleic anhydride resulted in increase in mechanical properties by20–35%.

[139]


Table 3: Continued.



7 Sisal Acetylationtreatment

Improvement in tensile strength and shear strength by 14.08% and 435%,respectively. 'e acetic acid treatment followed by ethyl acetate with H2SO4resulted in high levels of cellulose swelling or loosened cell wall structure.

[104]

8 Sisal Acetylationtreatment A decrease in dielectric constant with increasing frequency. [105]

9 Banana Acetylationtreatment

'e treated fiber has high fibrillation and is more rougher resulting in bettertensile properties than mercerization treatment. [109]

10 Grewia serrulata bast Acetylationtreatment

'e treated fiber has better dimensional stability and more moisture resistance.However, high degree of ultraviolent energy can degrade the composite. [142]

11 Phosphate bondedcomposite

Acetylationtreatment

'e treated fiber reduces water absorption and hence improves dimensionalstability, tensile strength, and stiffness. However, this treatment reduces the

impact strength as compared to other chemical treatments.[143]

Table 4: Effect of permanganate (KMnO4) treatment on various biofibers.

S.no.

Fibersused

Type of chemicaltreatment Effects Ref.

1 Sisal Permanganatetreatment Improvement in tensile strength. [75]

2 Oil palm Permanganatetreatment

'e treated fiber has highly fibrillated structure and hence very good fiber matrixadhesion. As a result, better tensile strength and modulus were observed [100]

3 Sisal Permanganatetreatment

At 1% concentration of KMnO4 polar groups between fiber and matrix are formedleading to degradation of cellulose. 'e hydrophilic tendency of fiber decreases as the

KMnO4 concentration increases up to an optimum.[105]


At 1% concentration, higher degradation of cellulose occurred due to formation of polargroup. Optimum properties were found to be better at 0.055% concentration. Tensile

properties were observed between alkali and peroxide.[144]


Improvement in interlaminar shear strength, tensile strength, and flexural propertiescompared to silane. But impact properties were lower than those of untreated fiber. [145]

6 Banana Permanganatetreatment

An increase in thermal diffusivity, tensile strength, and tensile modulus by 16%, 6.4%,and 7.5%, respectively. However, flexural strength and modulus were found to haveincreases of 6% and 10%, respectively, which were lower compared to alkali and silane

treatment.

[108]

7 Flax Permanganatetreatment

Improvement in tensile and moisture resistance as compared to alkali and silane-treatedfibers. [108]

8 Banana Permanganatetreatment

An increase in tensile strength and flexural strength by 5% and 10%, respectively.Increases in polarity and roughness of fiber were also observed. [146]

Table 5: Effect of peroxide treatment on various biofibers.



1 Sisal Peroxide treatment Enhancement in tensile properties. [72]

2 Sisal Peroxide treatment Crystallinity index and decomposition rate were found to be better at soaking time of30min. [146]

3 Kenaf Peroxide treatment At 30% fiber loading exhibited higher tensile and flexural strength, whereas modulus washigh at 40% fiber loading. [146]

4 Oil palm Peroxide treatment Improvement in flexural modulus as compared to other chemical treatments. [147]5 Sisal Peroxide treatment 50% higher tensile strength of treated fiber compared to untreated fiber composites. [105]

6 Jute Peroxide treatment'e treatment exhibited better tensile and flexural properties than alkali and

permanganate treatment, but not as superior as silane treatment, whereas the thermalstability was reduced.

[105]

7 Pineappleleaf Peroxide treatment Better in tensile strength, tensile modulus, and abrasion resistance as compared to

untreated composites. But there was a reduction in elongation breaks. [148]

8 Wheat straw Peroxide treatment 'e treated composites were found to have increase in ash content. Removal of ligninwas around 50%. [149]


fibers extracted from the waste of corn stalk. Initially thefibers were modified with different percentages of silaneconcentration such as 1%, 5%, 9%, and 13%. 'e effect ofsilane treatment on corn stalk fibers was investigated byFTIR and XRD analysis. Results showed improvement in thecrystalline size for 5% silane-treated fiber and also removalof hemicellulose, lignin, and other impurities present in thefibers was found by FTIR analysis. 'e impact behaviour of

the composites was also found to be superior for 5% silane-treated corn stalk fiber-reinforced composites. Finally SEManalysis revealed that the surface of fibers was rough incontrast with untreated fiber owing to the effect of silanetreatment. Liu et al. [177] investigated the effect of silanecoupling agent on the mechanical, tribological, and mor-phological characteristics of corn stalk fiber-reinforcedpolymer composites. 'e extracted corn stalk fibers were

Table 6: Effect of benzoylation treatment on various biofibers.

S.no.

Fibersused


1 Sisal Benzoylationtreatment Improvement in tensile strength by 91%. [75]

2 Jute Benzoylationtreatment Improvement in storage modulus and thermal stability. [150]

3 Flax Benzoylationtreatment

'e treated composites were found to have highest tensile and impact strength for LDPEand highest impact strength for HDPE. Resulted in less water absorption as compared to

silane and peroxide. Smooth fiber surface was observed.[43]

4 Sisal Benzoylationtreatment At 6% of benzoyl peroxide showed better mechanical properties. [83]

5 Banana Benzoylationtreatment

'e treatment significantly improved thermal conductivity and was found to haveincrease in tensile strength and modulus by 13% and 5%, respectively, although not as

good as alkali and silane-treated fiber.[109]

6 Sisal Benzoylationtreatment

'e treatment increased the activation energy for glass transition temperature (Tg).Maximum activation energy was observed. [151]

Table 7: Effect of acrylation treatment on various biofibers.

S.no.

Fibersused


1 Bagasse Acrylationtreatment

Acrylation treatment offers superior tensile and flexural strength compared to alkalitreatment. [152]

2 Oil palm Acrylationtreatment

'e treatment yielded higher extension and impact resistance. At 40% of fiber with50°C exhibited moderate level of moisture absorption compared to other treatments. [102, 155]

3 Flax Acrylationtreatment

'e treated fiber exhibited higher tensile strength and moisture resistance than thosetreated with silane, permanganate, and sodium chloride treatment. Higher smooth

fiber surface was observed.[131]

4 Flax Acrylationtreatment

At higher concentration, grafting was increased due to higher availability of monomermolecules in cellulose radicals as well as polymerization medium. [154]

5 Jute Acrylationtreatment An increase in tensile strength and flexural strength by 42.2% and 13.9%, respectively. [155]

Table 8: Effect of acrylonitrile grafting treatment on various biofibers.



1 Pineapple leaf Acrylonitrile grafting Improvement in tensile strength by adding AN (acrylonitrile grafting). It hasmajor effect on tensile strength. [75, 102]

2 Oil palm Acrylonitrile grafting'e treatment showed high strain rate and elastic modulus. But slight

improvement in stiffness was absorbed. However at 40 wt% of fiber with 50°Cexhibited higher moisture resistance compared to other treatments.

[153]

3 Agave Americanafibers Acrylonitrile grafting An increase in percentage of graft and decrease in the moisture resistance, in

addition to improvement in thermal stability of fiber. [156]

4 Sisal Acrylonitrile grafting 'e treatment showed enhanced tensile and flexural strength as compared toother treatments. Least degradability of fibers was observed. [157]

5 Pineapple leaf Acrylonitrile grafting 'e treatment possesses lower grafting yield than unmodified fibers. [158]

6 Cellulose polymer Graftcopolymerization Improvement in physical, chemical, and thermal resistance. [159]


exposed to surface modifications with silane couplingagents. Results showed an enhancement in water absorptionand porosity of the silane-treated corn stalk fiber-reinforcedcomposite specimens. Improvement in wear behaviour wasnoticed between the untreated and silane-treated corn stalkfiber-reinforced polymer composites. SEM analysis revealedthe formation of secondary plateaus on the compositespecimens which leads to the reduced wear rate on thecomposite samples. Jappes and Siva [178] researched theinfluence of silane modification done after the mercerizationtreatment on coconut sheath fiber to improvise the me-chanical properties of the composites. 'e fabricated

coconut sheath polyester composites were taken for testingof tensile, flexural, and impact strength, and the propertieswere compared with fabricated glass fiber-reinforced poly-ester composites. 'e coconut sheath fiber-reinforcedcomposites showed better mechanical properties againstglass fiber-reinforced polyester composites. 'e alkali andsilane modification done on the surface of coconut sheathimproved the hydrophilic nature of the fiber which ensuredenhanced adhesion between the coconut sheath fibril andthe matrix. SEM analysis was done to study the result ofalkalization on the coconut sheath fiber; dismissal of waxylayers and other surface related impurities to make the fiber

Table 9: Effect of isocyanate treatment on various biofibers.



1 Oil palm Isocyanatetreatment

At 40% fiber loading exhibited higher moisture absorption (280%) compared toother treatments. [153]

2 Sisal Isocyanatetreatment

'e treatment exhibited superior tensile properties than alkaline and untreatedfibers but with decrease in dielectric constant. [107, 146]

3 Pineapple leaf Isocyanatetreatment

'e treatment reduced hydrophilic tendency of the fiber compared to silane-treated composites. Reduction in % of crystallinity leads to increase in tensile

strength.[136]

4 Jute/hemp/flax

Isocyanatetreatment 'e treatment increased stiffness and reduction in impact by 17%. [31]

5 Fibrouscellulose

Isocyanatetreatment 'e treatment shows enhanced tensile, elongation, and interfacial adhesion. [160]

6 Kenaf Isocyanatetreatment

'e presence of isocyanate hydrolysis to urea, reacting with hydroxyl group offiber, decreases the moisture absorption and increases the mechanical properties. [72]

7 Sisal Isocyanatetreatment Improvement in tensile strength [161]

Table 10: Effect of maleic anhydride grafted treatment on various biofibers.


1 Jute Maleic anhydride grafted Coupling agent has greater effect on Young’s modulus and dynamicstorage modulus. [162]

2 Flax and hemp Maleic anhydride grafted 'e treatment improved dynamic and mechanical properties. [163]

3 WSF Maleic anhydride grafted 'e treatment significantly reduced crystallinity and thermal stability ishigher than that in acetylation treatment. [164]

4 Wood flour Maleic anhydride grafted An enhancement in tensile strength andmodulus properties became twiceas compared with untreated fibers. [165]

5 Sisal Maleic anhydride graftedImprovement in tensile, flexural, and impact strength by 50%, 30%, and58%, respectively. Reduction in water absorption by 61%. Higher level of

crystallinity was also observed.[166]

6 Banana/hemp/sisal Maleic anhydride grafted

Reduction in moisture resistance compared to untreated fiber. At 50%fiber loading, increases in tensile, flexural, and impact strength. Flexural

modulus values were higher than those of untreated fiber.[167]

7 Hildegardia Maleic anhydride grafted An increase in tensile properties with addition of compatibilizers. [168]

8 Pineapple leaf Maleic anhydride grafted'e treated fiber exhibited increased aspect ratio and matrix resulting inincreased tensile strength, impact strength, and flexural strength by 9%,

30%, and 3%, respectively, compared to untreated fibers[169]

9 Jute/sisal Maleic anhydride graftedmaleated HDPE

At 1% concentration of coupling agent increased the dynamic (storagemodulus and loss modulus) and static (tensile, flexural, and impact)

mechanical properties.[170, 171]

10 Wood flour Maleated polypropylene An increase in dimensional stability and strengthening by MAPPaddition. [172]

11 Natural fibers Maleated coupling agents An increase in interfacial adhesion between fiber and matrix due toremoval of hydroxyl group by addition of coupling agent. [173]


(a) (b) (c)

(d) (e)

Figure 1: 5% NaOH-treated alfa fiber at different time intervals: (a) raw fiber, (b) 2 h, (c) 4 h, (d) 6 h, and (e) 24 h [174].

(a) (b)

(c) (d)

Figure 2: SEM images. (a) Untreated Prosopis juliflora fiber. (b) 15% NaOH-treated fiber. (c) 10% NaOH-treated fiber. (d) 5% NaOH-treated fiber [175].


surface be rough enough has happened. Overall it wasconcluded that coconut sheath fiber-reinforced polyestercomposites could be a vital replacement for glass fiber-reinforced polymer composites. Kim et al. [179] studied theinfluence of bamboo fibers extracted by subjecting the fibersto steam explosion, alkalization, and chemical extraction.'e conversion rate from raw source to extracted fiber wasfound to be significant for alkali extractionmethod.'en theextracted fibers were subjected to alkali, silane, and com-bined treatment with different proportion to study theoptimal and suitable pretreatment type for bamboo fiber.'e tensile strength and modulus of mercerized bamboofibers were found to be superior when compared to silaneand alkali/silane modified bamboo fibers. But the me-chanical property of composites was found to be higher foralkali/silane-treated bamboo fiber-reinforced composites.Finally the water intake characteristics of alkali and alkali/silane-treated bamboo fiber-reinforced vinyl ester com-posites were better when compared to bamboo fiber-

reinforced vinyl ester composites. Liu et al. [177] exploredthe silane coupling agent’s influence on the tribologicalbehaviour of corn stalk fiber-reinforced polymer compos-ites. 'e silane treatment of fibril resulted in enhanced wearresistance; however, friction performance was not effective.'e examination of worn surface morphology revealed theemergence of secondary plateau on the composite surfacewhich enhanced the tribological characteristics of thecomposites. Lai et al. [180] investigated the possibility offabricating fiber-reinforced polymer composites using co-conut coir fiber as reinforcement material. 'e coconut coirfibers were exposed to mercerization followed by perman-ganate and stearic acid modifications to improve the ef-fective adherence between the fiber and matrix. 'e fibrilswere sized to 0.3mm and 0.5mm during the fabrication ofthe composites. Tensile and flexural strength results revealedthat when the percentage of fiber loading increased, thestrength values decreased.'is was due to the inability of thefiber to support the stress shifting from the polypropylene

0

20

40

TEN

SILE

STR

ENG

TH (M

Pa)

60

8070.81

53.11

G10K25T5 G10K20T10 G10K15T15SPECIMEN

G10K10T20 G10K5T25

68.0563.60

60.25

47.70 45.19 44.65

33.49

51.04

100

Alkali TreatedUntreated

(a)

0

50

FLEX

URA

L ST

REN

GTH

(MPa

)

100

150

200 181.08

144.86


G10K10T20 G10K5T25

154.04146.90 144.15

117.52 115.32 128.53102.82

123.23


(b)

0

20

60

40

IMPA

CT S

TREN

GTH

(KJ/m

2 )

80

120

100

96.00

79.68


G10K10T20 G10K5T25

95.00 91.00

48.00

75.53

39.8444.00

36.52

78.85

140


(c)

0

1

2

ILSS

(MPa

)

3

4

5

6

7

86.88

5.09


G10K10T20 G10K5T25

6.47 6.33 6.12

4.68 4.53

5.89

4.364.79


(d)

Figure 3: Tensile, flexural, impact, and interlaminar shear strength of glass/kenaf/tea leaf fiber-reinforced hybrid composites: effect of 5%NaOH alkaline treatment [12].


matrix and due to poor reinforcement property. Tensile andflexural modulus increased with increase in fiber loading,which was because of the higher fiber size, and this can alsobe inferred based on the aspect ratio of the fibrils. Zamanand Beg [119] evaluated the mechanical characteristics ofbanana fiber-strands-reinforced low density polyethylenematrix. 'e banana fiber strands were pretreated withmethylacrylate (MA) solution combined with methanol andbenzyl peroxide. 'e mechanical properties improved as aresult of better adherence between the interface of bananafiber strand and polyethylene matrix. 'e banana fiberstrands modified with starch solution showed improvementin composite properties against methylacrylate-treated fiber.Joseph et al. [161] studied the effects of benzoyl chloridetreatment on sisal fiber and found maximum thermal sta-bility compared to raw fiber composites. 'e treatmentremoves the hemicellulose and fatty substance in fibersurfaces for better mechanical and thermal properties. 'esisal/glass/filler/epoxy reinforced composites were used in

the frames, toys, and electronic panels. Sampathkumar et al.[181] analysed the influence of surface treatment on waterabsorption nature of Areca fiber. Due to the presence ofhydroxide and other constituents in chemicals, the waterabsorption nature was higher and this leads to poor wet-tability. 'e results of their work concluded that there was areduction in water absorption during acetylation of Arecafiber and increase in water absorption in treatment withalkali. Alfa fibers were kept under the various fiber surfacetreatments involving acetylating and it was confirmed thatthe treatment enhanced the resistance of fiber to moisture[182, 183]. Bisanda [184] reported that alkali-treated sisalfiber-reinforced polylactic acid composites removed ligninand waxy substances which led to better mechanicalinterlocking. 'e tensile, flexural, impact, and interlaminarshear strength (ILSS) of 5% NaOH-treated kenaf and tea leaffibers-reinforced composites improved by 33.32%, 25%,20.48%, and 35.16%, respectively, when compared withuntreated composites due to removal of hemicellulose,

(a) (b)

(c) (d)

(e)

Figure 4: SEMmicrographs of (a) tensile test of 5% NaOH-treated composites, (b) matrix microcrack, (c) fiber pull-out and void formationin the impact test specimen, (d) fractured surface of impact test specimen, and (e) hollow structure of natural hybrid fiber composites [12].


lignin, pectin, and waxy elements which resulted in betterinteractions between hydrophilic fiber and hydrophobicmatrix [12]. Figure 3 presents the mechanical properties ofglass/kenaf/tea leaf fiber-reinforced hybrid composites.

'e SEM analysis (Figure 4) shows that mechanicalproperties of untreated fiber composites were decreased bythe problems such as fiber breakage, fiber pull-out, for-mation of micro voids, and nonuniform distribution of fiberand matrix. 'e 5% NaOH-treated kenaf and tea leaf fiberimproved the adhesion between the fiber and matrix whichresulted in better mechanical properties [12].

'e biodegradability of chemically modified cellulosicfibers is associated with physical, chemical, mechanical, andthermal properties [5, 185, 186].'e agricultural waste fiber-reinforced composites are used in automobile, aerospace,construction materials, packaging applications, and medicalapplications. Sharma and Kumar [187] studied the tensileproperties of sugar palm fiber obtained from different height

of the palm plant. 'e tensile properties enhanced in thebottom part of the tree fiber due to their chemical com-position, particularly cellulose, hemicellulose, and lignin[188, 189].

'e tensile strength and tensile modulus of 5% NaOH-treated jute fiber-reinforced polyester composites improvedby 5.2% and 17.2%, respectively, when compared withuntreated composites due to better interaction between fiberand matrix (Figure 5). It was also revealed that 5% NaOH-treated hemp fiber-reinforced PLA composites exhibitedoutstanding results compared to silane-treated hemp fiber-reinforced PLA composites [190]. 'e tensile strength andtensile modulus of 5% NaOH-treated hemp fiber-reinforcedPLA composites improved by 9.9% and 14.1%, respectively,when compared with silane-treated composites owing tobetter removal of unwanted substances such as hemicellu-lose and waxy elements. Likewise, 5% NaOH-treated coirfiber-reinforced epoxy composites exhibited superior tensile

40

3030.6

34.2

3.36 3.94

20

10

0

Jute/Polyester5% NaOHUntreated

Tensile Strength (Mpa)Tensile Modulus (Gpa)

(a)

80

60

77.570.5

8.9 7.8

40

20

0NaOH

Hemp/PLASilane


(b)

25

20

23.8

20.2

2.96 2.77

15

10

0NaOH

Coir/EpoxyUntreated


(c)

Figure 5: Tensile properties of 5% NaOH-treated jute/polyester and hemp/PLA composites [190].


properties such as tensile strength (17.8%) and tensilemodulus (6.8%) compared to untreated fiber composites.Based on various chemical treatments, alkaline treatment(5% NaOH) is the most economical and effective treatmentin promoting better communications between fiber andmatrix by removal of hemicellulose, lignin, and waxy ele-ments due to disruption of hydrogen bonding in the fiberstructure, thus resulting in better mechanical and thermalproperties.

4. Conclusions

In this detailed review, the effects of various chemicaltreatments on different biofiber-reinforced composites weresummarized. Furthermore, the effect of constitution ofbiofibers was also reported. 'e physical, mechanical, andthermal properties of various biofibers-reinforced com-posites were improved up on modification of fiber surfaces,while fiber swelling effect and water absorption rate weredecreased by various chemical treatments like alkaline, si-lane, acetylation, permanganate, peroxide, benzoylation,acrylonitrile grafting, maleic anhydride grafted, acrylation,and isocyanate. Based on various chemical treatments, al-kaline treatment (5% NaOH) is the most economical andeffective treatment in promoting better communicationsbetween fiber and matrix by removal of hemicellulose,lignin, and waxy elements due to disruption of hydrogenbonding in the fiber structure, thus resulting in bettermechanical and thermal properties. It was concluded thatalkaline treatment of fibers with 5 wt% NaOH made thefibers more resistant to deformation and heat. 'e alkalinetreatment has been one of the successful methods used totreat the natural fibers in order to achieve better results.

Data Availability

'e data used to support the findings of this study are in-cluded within the article.

Conflicts of Interest

'e authors declare that there are no conflicts of interestregarding the publication of this article.

Acknowledgments

Alagar Karthick gratefully acknowledges group FQM-383from Universidad de Cordoba, Spain, for the provision of anhonorary visiting research position in the group.'is projectwas funded by a Researchers Supporting Project (no. RSP-2021/405), King Saud University, Riyadh, Saudi Arabia.

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[2] S. Sathish, L. Prabhu, S. Gokulkumar, N. Karthi, D. Balaji,and N. Vigneshkumar, “Extraction, treatment and applica-tions of natural fibers for bio-composites—a critical review,”International Polymer Processing, vol. 36, no. 2, pp. 114–130,2021.

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